Stock your car came with exhaust manifolds, not really headers. Anyhow, at least the front of the X-pipe should bolt right up to the stock exhaust. Not sure about your cat-back.
There is no need to weld it, it comes with a ball joints, just make sure you torque it down pretty good, the clamps that hold the X pipe together you just put some high temp silicon and tighten it, don't over torque the clamps